1. Field of the Invention
The present invention relates to a method for transmitting a power headroom report and a buffer status report for a user equipment in a wireless communication system and related communication device, and more particularly, to a method for transmitting a power headroom report and a buffer status report according to a status of an uplink transmission buffer of a user equipment and related communication device.
2. Description of the Prior Art
A long-term evolution (LTE) system, initiated by the third generation partnership project (3GPP), is now being regarded as a new radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) and communicates with a plurality of mobile stations, also referred as user equipments (UEs). The LTE radio protocol stack includes the Layer 3, also known as the Radio Resource Control (RRC) layer, the Layer 2, consisting of three sub-layers that are the Packet Data Convergence Protocol (PDCP) layer, the Radio Link Control (RLC) layer, and the Medium Access Control (MAC) layer, and the Layer 1, also known as the Physical (PHY) layer.
A dynamic scheduling function is used in the UE MAC layer and the eNB MAC layer for utilizing shared channel (SCH) resources. For uplink (UL), an eNB allocates an UL resource to an UE through a physical downlink control channel (PDCCH) signaling. In order to utilize SCH resources more efficiently, a semi-persistent scheduling (SPS) function is further introduced in the LTE system for serving upper layer applications that generate semi-static size data periodically, e.g. VoIP services. The eNB activates SPS resources by sending a PDCCH signaling, and after the SPS activation, SPS resources are periodically allocated to the UE without being through the PDCCH signaling.
According to the UE MAC specification, a Buffer Status Report (BSR) provides the serving eNB with information about the amount of data available for transmission in an UL transmission buffer of the UE. The BSR is one of MAC control elements included in a MAC Protocol Data Unit (PDU). There are three types of BSRs, regular BSR, periodic BSR and padding BSR, caused by different triggering events. The regular BSR is triggered when UL data which belongs to a logical channel with higher priority than those for which data already existed in the transmission buffer arrives at the transmission buffer, when a serving cell change occurs, or when a retransmission BSR timer expires and the UE has data available for transmission. The padding BSR is triggered when UL resources are allocated and the number of padding bits is equal to or greater than the size of a BSR MAC control element. The periodic BSR is triggered when a periodic BSR timer expires, and is used for providing the eNB with updated status of the UE transmission buffer periodically. The regular BSR, the periodic BSR, and the padding BSR are reported by different formats including long BSR, short BSR, and truncated BSR.
In addition, a Power Headroom Report (PHR) provides the serving eNB with information about the difference between the nominal UE maximum transmit power and the estimated power for UL-SCH transmission, which is also one of MAC control elements included in a MAC PDU. Please refer to FIG. 1, which is a diagram illustrating the format of a PHR MAC control element according to the prior art. The PHR MAC control element consists of 2-bit “R” field for reserved bits and 6-bit “PH” field indicating power headroom level. Each reserved bit is set to 0.
The PHR is triggered when a prohibit PHR timer expires and the path loss has changed more than a specific value since the last transmission of a PHR when the UE has UL resources for new transmission, and is also triggered when a periodic PHR timer expires. When the UE has UL resources allocated for new transmission in the current TTI, and at least one PHR has been triggered since the last transmission of a PHR, and the allocated UL resources can accommodate a PHR MAC control element, the UE MAC layer obtains the value of the power headroom from the UE PHY layer, generates a PHR MAC control element based on the value reported by the UE PHY layer, and instructs the UE PHY layer to transmit the PHR MAC control element. Based on the received PHR, the eNB adaptively selects a modulation and coding scheme (MCS) for the UE, and therefore the UE is able to use an appropriate data rate to transmit data for higher transmission efficiency. According to the current MAC specification, the UE MAC layer performs logical channel prioritization to generate a MAC PDU, and the BSR MAC control element has higher priority than the PHR MAC control element.
Note that, when SPS is configured, BSRs are necessary to quickly signal to the eNB the possible occurrence of data that has lower priority than SPS data in the UL transmission buffer. However, BSRs do not help in quickly adapting the MCS for the UE, which is achieved through PHRs. In the case of SPS data transmission, such as VoIP transmission, the transport block size agreed for the E-UTRAN allows a short BSR or a PHR to be transmitted. For VoIP transmission, with every new speech frame, a regular BSR is triggered and sent in an UL resource, and most of BSRs report empty buffer, referred as empty BSRs.
Based on that the BSR has higher priority than the PHR, with every speech frame, no PHR can be sent without segmentation or delay even the BSR reports empty UL transmission buffer; however, the empty BSR seems less important than the PHR. In this situation, the eNB may lose the opportunity to adapt the MCS for the UE promptly due to the late PHR even no PHR. Furthermore, when the UE decides not to segment the VoIP packet by considering logical channel prioritization, the PHR is delayed for a long time, which also decreases transmission efficiency. Therefore, in order to have efficient transmission, it is important for the UE to send the BSR and the PHR when they are surely required.